Unable to purchase Puzze Agent + TOMI season Upgrade

I'm trying to buy Puzzle Agent, and with the 50% off offer, I'm trying to buy the Monkey Island season upgrade - I already bought episode one which I gifted to a friend, and want to complete his season. Figured I might as well do it while it was cheaper for me. :)

BUT, the store won't let me put the season upgrade in the cart - no add to cart button..! It says I qualify and that's all.. plus, it's being gifted to someone I gifted episode 1 to (or it will be if I can get it in the cart).

I'm not trying to buy the sam and max + tomi bundle, nor the dvd set... just the $26 season upgrade.